(tables S1 and S2). The individual NPs have a
nearly spherical shape, with a metal core diameter of 2.2 nm and an overall diameter (including
the ligand shell) of 3.3 nm (fig. S4).

In contrast to conventional spherical NPs,
which are typically packed into superlattices
with simple translational symmetry, such as face-centered cubic (fcc) or body-centered cubic (bcc)
(11, 17), the Au246(p-MBT)80 NPs packed into a
more complex monoclinic lattice (Fig. 1). In the
(001) plane of the crystal lattice, the NPs organized into a square lattice (Fig. 1A), akin to the
packing mode in the {100} plane of the fcc lattice.
The interparticle distance of 3.1 nm (less than the
3.3 nm overall size of the NP) arose from ligand
interlocking. These two-dimensional (2D) lattices
stacked up obliquely, instead of perpendicularly
along the z direction (Fig. 1B), so the overall
3D lattice deviated from the fcc lattice. The NP
packing density of ~60% is less than the 74% of
the fcc lattice. Such monoclinic packing should
be driven by specific interparticle interactions,
because entropy alone would favor close packing
(5–9). Indeed, when zooming into the surfaces
of the NPs, the monoclinic packing was correlated
to the alignment of surface ligands among NPs
(Fig. 1, D to F, green, blue, red).

The p-MBT ligands were highly ordered andself-organized into two different patterns on thegold sphere (Fig. 2A). At the pole site of thegold sphere, 25 of the p-MBTs were rotationallyarranged into four pentagonal circles (Fig. 2B,highlighted in red, blue, blue, green). Each circlehad the same “latitude” and rotational direction.

Instead of “on-top” adsorption, the thiolates (
characterized by S-C bond vectors) “tilted” away from
the radial direction of the gold sphere because
of the gold-thiolate binding geometry (18). This
pattern we call a-rotation created a “singularity”
(19) at the pole (Fig. 2B). At the waist site, six of
the p-MBT ligands were aligned into three alternating parallel pairs to form a pattern we
call b-parallel (Fig. 2C). Five of these b-parallel
patches circled up and covered the waist of the
NP. The packing densities of ligands are ~14
ligands nm–2 for a-rotation and ~6 ligands nm–2
for b-parallel as measured based on the surface
area of the inner gold sphere (Fig. 2A, magenta
polyhedron). The clockwise and counterclockwise
rotational arrangement of p-MBT ligands induces
chirality in the NP, and both chiral isomers (
denoted R/L) participate in the crystal packing (Fig.
1, C and F). The NPs with the same chirality are
packed in the same square layer, and the neighboring square layers are composed of NPs with
opposite chirality (Fig. 1C).

Such rotational and parallel self-assembledsurface patterns of ligands are reminiscent ofthe a helix and b sheet in proteins, which sug-gests that NPs could exhibit a level of structuralcomplexity comparable to that of biomolecules.The secondary structures of proteins are mainlystabilized by the hydrogen bonds. Here, the sur-face patterns on the NPs are stabilized by inter-molecular C-H⋅⋅⋅p interactions, in which the C-Hbonds from the phenyl rings or the methyl groupsinteract with the p electrons (Fig. 2D). Suchintermolecular interactions were observed in thepacking structures of aromatic molecules andsupramolecules, and the strength is about 1.5 to2.5 kcal mol–1 (20, 21). Specifically, the C-H⋅⋅⋅pinteractions in the a-rotation linked the 25 p-MBTligands into five spirals, with the H⋅⋅⋅p distancesranging from 2.5 to 3.0 Å and C-H-p angle rangingfrom 112° to 147° (fig. S5). For the b-parallels, thealternating pattern among the three pairs wasalso stabilized by the C-H⋅⋅⋅p interactions (fig. S6).Within each parallel pair, the phenyl rings wereoffset to avoid the repulsion between p electrons.We reason that the surface patterns are intrinsic toNPs instead of being induced by crystallization,because the collective C-H⋅⋅⋅p interactions can gen-erate an energy barrier and stabilize the pattern,similar to the case in which the hydrogen bondscan stabilize the DNA double helix in solution.To study the interparticle interactions, weisolated the coordination environment of NPs

R

L
R
L

y

xzyxz

R
L
R
L

Fig. 1. Packing structure of the derivatized Au NPs in single crystals. (A to C) View from z direction (A), y direction (B), and x direction (C). Magenta,
blue: Au NPs with different chirality; yellow: sulfur; gray: carbon. (D to F) Alignment of surface ligands among the NPs. Gray: ligands located at the waist
of the NP; red, blue, green: ligands located at the poles.